1. Field of the Invention
The invention relates to a filter cartridge for recovery of low boiling point halogenated hydrocarbons, in particular, for recovering inhalation anesthetics from patient exhalent.
2. Discussion of the Background
Volatile anesthetic gases such as halothane, sevoflurane, enflurane, isoflurane and desflurane are frequently used in medical practice. These gases are chlorofluorocarbons (CFC) or hydrofluorocarbons (HFC). They will usually be entirely released into the environment during or after their use for anesthesia. This can be very harmful to patients and likely to medical personnel. In addition, such volatile anesthetic gases possess a potential to damage the environment, for example in the form of “ozone layer hole degradation” or “greenhouse effect”. An estimation referring to the member states of the European Union showed that in 1995 alone, a load of the atmosphere of approximately 700 t arose from inhalation anesthetics. This quantity corresponds to 0.25% additional loading of the environment with carbon dioxide [Zeitschr. Anästhesiologie and Intensivmed. 6 (39), 301-306, 1998].
In the state of the art describing elimination of halogenated hydrocarbons from carrier gas, adsorption filters are used which function on the basis of micro-porous sorption material, such as activated charcoal and zeolites, for the purpose of temporary retention and storage of the gases. Reactive activated charcoal is already suitable for the cleaning of process of exhaust air (DE 37 13 346, DE 39 35 094 and DE 40 03 668). The conditions for a high sorption capacity of the sorption devices, connected with optimal regenerative power are already stated in the DD 239 947, DE 36 28 858 and DE 37 31 688. The recovery of halogenated hydrocarbons can take place economically with a high degree of recovery via desorption only under high temperatures and low pressures conditions. However, as a result of thermal treatment structural damages of the sorbent and also the formation of halogenated decomposition products of halogenated hydrocarbons arise.
In DE 37 13 346 and DE 195 49 271 the removing of halogenated hydrocarbons by means of zeolites is described. Zeolites are also particularly suitable for removing of pollutants from aqueous solutions (DE 44 06 766 and DE 195 31 933). The sorption of halogenated hydrocarbons is mentioned in DE 42 33 577. Recently, aluminum-poor and dealuminated zeolites have been used as sorbents (DE 195 32 500). Their alumina part (aluminium oxide) is substantially replaced by silicon dioxide. For example, dealumination of Na—Y-zeolites of the Wessalith DAY results in a favorable pore opening of 7.4 Å for the sorption of inhalation anesthetics. Zeolites exhibit a high thermal stability and a small catalytic activity for the formation of toxic products of halogenated hydrocarbons. The sorption of water to these so-called hydrophobic zeolites is noticeably reduced in favor of the sorption of halogenated hydrocarbons. A well-known procedure for the separation and recovery of inhalation anesthetics (DE 42 08 521) concerns their adsorption at activated charcoal or a zeolite filter with exception of the accompanying N2O (laughing gas), in which the remaining further carrier gases are afterwards supplied to a catalytic post combustion. The temperature of 550° C. that is required for this purpose is still uneconomically high. The recoverable active substances are irreversibly withdrawn from the device. A draw-back is that the narrow pores of the activated charcoal with a wide pore spectrum permanently adsorb halogenated hydrocarbons which can only be set free at high temperatures. Also, during the recovery of inhalation anesthetics (DE 43 08 940 and DE 195 49 271), the temperatures required for desorption in gas phase, 100° C. to 200° C., still lead to medically dangerous by-products.
One of the methods for recovering halogenated hydrocarbons from a gas stream (EP 0 284 227; CA 1 339 833) is using a hydrophobic zeolitic molecular sieve adsorbent with a narrow range pore distribution, which selectively separates the active substances of the group of halogenated ether from higher hydrocarbons. The desorption takes place in a container by means of a nitrogen purging gas stream at 30° C. to 150° C. The anesthetics are then condensed and recovered. However, a temperature range between 30° C. to 40° C. was not yet sufficient for an economically efficient degree of recovery. On the other hand, temperatures over approximately 140° C. lead to staining of the sorbent particles due to structural changes and development of coke deposition. Also, the spatial separation of the adsorption and the recovery cycles do not yet correspond to the economic requirements of a cycle of the exhalation gas.
In a method and device for the recovery of gases (DE 197 49 963; WO 99/22845), some anesthetics components are bound to the adsorbent, while others pass through it. In this method, dealuminated zeolites adapted to the process are already used favorably as sorbent material. Through thermal treatment, the adsorbed gas will be desorbed easily and condensed in a condenser and purified for reuse. Due to the high vapor pressure of the anesthetics, the condensation must take place within the temperature ranges of 2° C. to 8° C. The desorption of isoflurane takes place under vacuum (approx. 10 mbar) and under simultaneous heating at for instance 100° C. to 160° C. Thereby, the maximum desorption temperature is about 60° C. lower than that required for activated charcoal. Desflurane is desorbed between 90° C. and 130° C. Its low pressure, however, favors disadvantageously the deposit of coke-like materials due to the absence of oxidizable gas components.
In DE 101 18 768, a method is described concerning a gentle recovery of zeolites from a filter cartridge by a steam distillation of the sorbents. Modified and/or dealuminated zeolites with low water adsorption capacity below Ma-2% cause lowering of the desorption temperature which is favorable for the sorbents. Under normal pressure, a temperature limit is preferably 100° C. Additionally sorbed fractions of anesthetics are set free through the additional extraction. The dephlegmation of the rising gases leads to a partial condensation and to the return of the mixture into the evaporator with water as a main fraction. A further cooling of the gases leads to the development of a layer-like pre-separated mixture in a settling container. The specifically light-density water layer is recycled into the evaporation process, while the heavier-density layer from a post-purification is supplied for repeated use of the anesthetics. Other possible degradation products accumulate in the water layer. Thus, the developed harmful products remain in the water circulation and can be periodically removed.
Concerning adsorption and desorption of inhalation anesthetics, conventional filter cartridges with zeolites exhibit different characteristic parameters that depend substantially on flow and temperature conditions. In order to reach a standardization of the processing without a time delay, for example, a different energy supply from the outside into a cartridge is arranged, whereby the adsorbent anesthetics can be set free from the inside of the cartridge without time delay (EP 0 611 174, EP 1 222 940). Also, specially formed embodiments of filter cartridges for inhaled gases are usual in order to evenly use the fillings even at higher flow rates (DE 36 12 924) and to avoid local break-through of the adsorbents through the filter layer.
It is well-known from chemical engineering, that a cross-flow of gaseous phase and stationary sorbent is indicated when a sorptive is to be removed from the phase dispenser to a large extent. On the other hand, the loaded sorbent can be regenerated gradually with a changed regeneration fluid in the cross-flow cascade without a time delay.
Finally, domestic and commercial zeolites containing filter cartridges are neither suitable nor designed for a recovery of anesthetics by means of water vapor.